In recent years, a low fuel consumption is eagerly desired for automobile tires in view of an environmental problem and a resource problem. Further, enhancements of wet grip performance and abrasion resistance are desired from viewpoints of safety and durability, respectively.
A rubber composition having silica incorporated therein exhibits reduced heat build-up as compared with a rubber composition having conventional carbon black incorporated therein. Therefore, the silica-incorporated rubber composition gives tires exhibiting a low fuel-consumption.
However, silica usually has poor affinity to carbon black, and therefore, a vulcanizable rubber composition having silica incorporated therein has poor processability and exhibits insufficiently reduced heat build-up and insufficient abrasion resistance. Therefore a silane coupling agent is usually incorporated in the rubber composition. However, even when a silane coupling agent is incorporated, the abrasion resistance is occasionally still below a satisfactory level as compared with a carbon black-incorporated rubber composition. A silane coupling agent is expensive and thus the incorporation of a large amount thereof leads to an increase in cost.
Attempts of modifying a rubber itself for enhancing its affinity for silica are being examined. For example, a rubber composition comprising a polymer rubber and silica, which rubber has been obtained by lithionizing a diene polymer rubber with an organic lithium compound and then allowing the lithionized diene polymer rubber to react with a silicon-containing compound, is described in Japanese Unexamined Patent Publication (hereinafter abbreviated to as “JP-A”) No. H10-7702. A rubber composition comprising a diene polymer having a silanol group, and special carbon black particles having silica bonded to the surfaces thereof is described in JP-A H10-316800.
The above-mentioned rubber compositions comprising a modified diene polymer rubber give crosslinked rubbers exhibiting reduced heat build-up, but, uncrosslinked silica-incorporated rubber compositions thereof have poor processability and are liable to give crosslinked rubbers exhibiting poorly balanced wet grip performance and abrasion resistance.
A rubber composition comprising a polyorganosiloxane-modified diene polymer and silica is described in JP-A H9-110904, wherein the polyorganosiloxane-modified diene polymer is obtained by preparing diene polymers having an active alkali metal-bonded terminal by polymerization using an alkali metal polymerization initiator, and then, allowing the thus-prepared diene polymers to react with 0.1 to 2 moles, per mole of the alkali metal polymerization initiator used, of a polyorganosiloxane having specific functional groups. It is to be noted, however, that the factual data showing the effect of the invention have been prepared only on a modified diene polymer obtained by allowing the diene polymers to react with one mole of the polyorganosiloxane per mole of the alkali-metal polymerization initiator used.
A rubber composition comprising a silsesquioxane-modified diene polymer and silica is described in JP-A 2002-80534, wherein the silsesquioxane-modified diene polymer is obtained by preparing diene polymers having an active alkali metal-bonded terminal by polymerization using an alkali metal polymerization initiator, and then allowing the thus-prepared diene polymers to react with 0.1 to 1.5 moles, per mole of the alkali metal polymerization initiator, of a silsesquioxane compound having a polyhedron structure. It is to be noted, however, that the factual data showing the effect of the invention are given in the patent publication only on modified diene polymers obtained by allowing the diene polymers to react with 0.5 to 1.2 moles of the silsesquioxane compound per mole of the alkali-metal polymerization initiator.
The above-mentioned polyorganosiloxane-modified diene polymer and silsesquioxane-modified diene polymer give crosslinked rubbers exhibiting well-balanced reduced heat build-up and wet grip performance as compared with a dimethyldichlorosilane-modified diene polymer, but, silica-incorporated uncrosslinked rubber compositions prepared from these modified diene rubbers have poor processability and occasionally give crosslinked rubbers having poor abrasion resistance.